The US opened new fronts in its fight against WikiLeaks today as civil rights groups accused the authorities of censorship.

The whistleblower's website went offline for the third time in a week this morning – the biggest threat to its online presence so far. The site re-emerged later on a Swiss domain.

France joined international calls for WikiLeaks to be closed down, warning that it was "unacceptable" for a "criminal" site to be hosted in the country.

The moves came only days after Amazon pulled the WikiLeaks site from its servers after political pressure from Joe Lieberman, chairman of the Senate homeland security committee.

Lieberman is not finished with Amazon, and is planning to write to the organisation within the next 24 hours asking for details of its relationship with WikiLeaks. The issue is fast turning into a row over freedom of speech, as Democratic and Republican politicians joined calls for action against WikiLeaks, including emergency legislation for legal challenge.

Liberal activists saw echoes of the row involving China and Google earlier this year, censorship the Obama administration decried at the time.

The US civil rights group Human Rights First wrote to Amazon saying that its decision to cease hosting WikiLeaks raised serious concerns and asked the book group to consider this before responding to Lieberman's request for more information.

Rainey Reitman and Marcia Hofmann, of the Electronic Freedom Foundation, which campaigns for internet freedom, writing on the organisation's site, said it was "unfortunate that Amazon caved in to unofficial government pressure to squelch core political speech. Amazon had an opportunity to stand up for its customer's right to free expression. Instead, Amazon ran away with its tail between its legs".

There have been calls on blogsites for a boycott of Amazon.

Leslie Phillips, communications director for the Senate homeland security committee, disputed any parallel with China's censorship of the internet. "It is not at all the same," she said. "In China, there is a fiat from above."

Lieberman, she said, does not have the authority to shut down Amazon or tell it who its clients should be.

She said Lieberman is to write to Amazon asking for basic facts such as when it first realised that WikiLeaks was disseminating classified information.

In a blogpost on Thursday night, Amazon denied giving in to political pressure. It said WikiLeaks was violating its terms of service, which included a provision that the content should not be harmful. "It is not credible that the extraordinary volume of 250,000 classified documents that WikiLeaks is publishing could have been carefully redacted in such a way as to ensure that they weren't putting innocent people in jeopardy," Amazon said.

Lieberman and other senators are to introduce legislation that they have named the Shield Act that would allow the administration to go after WikiLeaks. But the bill stands little chance of passage as it would probably go not to the homeland security committee but the Senate judiciary committee, which is headed by Patrick Leahy, a Democrat and long-time champion of liberal issues.